The present invention is based on a boat propulsion unit of the type having a propulsion propeller, which is arranged under the boat's bottom, which is preferably essentially flat, at least in the area of the propulsion unit, and which can be withdrawn into a sleeve to a position above the plane of the boat's bottom.
More specifically, the present invention is based on a boat propulsion unit which includes a rudder propeller. Such propulsion units are associated, in general, with watercraft which are intended for traveling in shallow waters. Such watercraft have a bottom, at least parts of which are flat, and a vertical propeller steering shaft which connects the engine in the interior of the boat to the propeller under the boat's bottom and outside the watercraft, and passes through the bottom, while being sealed against water. The propeller causes propulsion of the watercraft by rotation around its horizontal axis. Being a rudder propeller, it is not only rotatable around its horizontal axis, but it is also pivotable around the longitudinal axis of the propeller steering shaft in order to change or otherwise determine the direction of travel of the watercraft.
The possibility of traveling in shallow waters is limited when the propeller is located under the boat's bottom. Therefore, the propeller can be withdrawn for such cases into a sleeve integrated within the boat's contour. When the watercraft reaches correspondingly shallow waters, the propeller is pulled up into the sleeve to the extent that it comes to lie above the plane of the boat's bottom. Even though the watercraft can no longer be steered by means of the retracted propeller, it nevertheless can still travel, via other means, in an area through which it could not otherwise navigate because of the depth of water being excessively low.
The problem thus far described, is one pertaining to navigation. The watercraft should be able to enter very shallow waters and be moved therealong temporarily, e.g., for being loaded or unloaded at the shortest possible distance from the shore.
The present invention deals with another aspect of the problem. It is directed to modification of such a watercraft, whose propulsion system is designed according to the foregoing requirements, such that repairs can be carried out on the underwater part of the propulsion system at the lowest possible expense. In prior-art watercraft, repair on the outboard part of the propulsion system has always made it necessary to bring the watercraft on land or into a dock. This was the prerequisite for a repair unless one wished to remove the entire propulsion system, which involved considerable expense.